Photographic processes using silver halide have been most widely used hitherto because they are excellent in photographic properties such as sensitivity or control of gradation, etc. as compared with other photographic processes, for example, an electrophotographic process or a diazo photographic process. In the recent years, with respect to the image formation process for the photographic materials using silver halide, many techniques capable of obtaining images ease and rapidly have been developed by changing the conventional wet process using a developing solution into a dry process such as heating, etc.
The heat-developable photographic materials are known in the field of these technique, and the heat-developable photographic materials and processes therefor have been described in U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108 and 1,167,777, and Research Disclosure No. 17029, p.p. 9 to 15 (June, 1978).
As processes for obtaining color images, many processes have been proposed. With respect to processes for forming color images by the reaction of an oxidation product of a developing agent with coupler, it has been proposed to use a p-phenylenediamine type reducing agent and phenolic coupler or active methylene coupler as described in U.S. Pat. No. 3,531,286, p-aminophenol type reducing agent as described in U.S. Pat. No. 3,761,270, sulfonamidophenol type reducing agent as described in Belgium Pat. No. 802,519 and Research Disclosure, p.p. 31 and 32 (September, 1975) and combinations of sulfonamidephenol type reducing agent and 4-equivalent coupler as described in U.S. Pat. No. 4,021,240. These processes, however, have a disadvantage in that color images formed are turbid, because a reduced silver image and a color image are formed at the same time on the exposed area after heat development. Examples of method designed to eliminate this drawback are a process which comprises removing the silver images by liquid processing, for example, a bleach-fix processing, or a process which comprises transferring only dyes to a sheet having another layer, for example, an image receiving layer. However, this process is not desirable because it is not easy to transfer only the dyes by distinguishing them from unreacted substances.
Another known process comprises introducing a nitrogen containing heterocyclic group into dyes, forming silver salts and releasing the dyes by heat development. This process is described in Research Disclosure No. 16966, p.p. 54 to 58 (May, 1978). According to this process, clear images can not be obtained, because it is difficult to control the release of the dyes from unexposed areas, and thus it is not a conventional process.
With respect to another known process for forming positive color images by a thermal silver-dye bleach process, useful dyes and a means for bleaching have been described in Research Disclosure No. 14433, p.p. 30 to 32 (April, 1976), and No. 15227, p.p. 14 to 15 (December, 1976), and U.S. Pat. No. 4,235,957. However, this process requires an additional step and an additional material for accelerating bleaching of dyes, for example, heating with a superimposed activating agent sheet. Furthermore, the process is not desirable because the resulting color images are gradually reduced and bleached by coexisting free silver when preserved for a long period.
Still another process for forming color images which utilizing leuco dyes has been described in, for example, U.S. Pat. Nos. 3,985,565 and 4,022,617. However, this process is not desirable because it is difficult to incorporate stably the leuco dyes in the photographic materials and coloration gradually occurs during preservation.